1. Field of the Invention
This invention relates to an ink jet recording method and a color ink jet recording device which are applied to copying machines, facsimiles, printers for image processing, and other printers in general. More particularly, it pertains to a scanning type ink jet recording device equipped with a plural number of recording heads having a plural number of discharge ports arrayed in the sub-scanning direction and to a recording method of said device.
2. Related Background Art
In the prior art, for improvement of gradation or density of recorded images, it has been practiced to form a plural number of picture elements for forming the image on a recording medium with a plural number of overlapping ink droplets, or for improvement of adjacent picture element densities, to shoot a plural number of ink droplets at a high density. When this is applied to color, there is the advantage that color reproducibility can be improved.
Further, in the case when the recording medium is a transparent film for transmission type OHP (overhead projector), etc., the image recorded thereon is required to be increased in transmission density as a whole, and also in this case the above system is effective.
In these systems, emission of ink droplets for emitting a plurality of ink droplets on one picture element is performed by scanning of the recording head for plural times while the recording medium is fixed as such. Accordingly, scanning for plural times corresponding to higher densification or scanning for plural times corresponding to the ink colors to be mixed is required for the recording range per one scanning in scanning type recording, and no high speed recording can be practiced. In other words, if a high density image or a color image is formed for ordinary printing, a printing time is required which is as long as 2-fold to 4-fold of the time required for ordinary printing.
Accordingly, for the purpose of making faster even by little the recording speed, in order to increase the recording range per one scanning, it is effective to enlarge the recording head by formation of a multi-nozzle, but since it is limited to make the head lengthy, the ratio of shortening of the printing time is little in scanning type ink jet recording.
In contrast, U.S. Pat. No. 4,320,406 discloses a constitution which miniaturizes the head constitution itself of a color device by dividing the nozzles (discharge port) of the single recording head itself formed into a multi-nozzle into four nozzle units predetermined for different inks, but since the recording speed depends on only one recording head, the ink discharge range of one color is reduced to great extent, whereby the advantage of the recording formed into a multi-nozzle will be lost.
The present inventors have investigated about the prior art and found the following problems.
When image density is enhanced or color formation is effected by scanning for plural times continuously for the recording range to be recorded by one scanning, because the picture element is formed by ink discharge for plural times, the ink amount per one picture element becomes larger, and therefore in the case of recording medium of which ink receptive amount is limited such as film for OHP or one having poor ink absorptivity, the amount or the rate of ink absorption is limited, whereby ink may be sometimes overflowed from the picture element, resulting in deterioration of the image quality.
Particularly, a plural number of recording heads corresponding to the respective inks had a constitution arranged in the scanning direction, and therefore in the case when different inks were overlapped, the emission density per unit time became greater, whereby the above problem of ink overflow became marked.
Generally speaking, recording papers with increased ink absorption amount have been known, and recording is possible only on expensive and specific converted papers and only for limited recording. However, when users employ another paper or sheet, defective recording may be brought about, and in the worst case, users sometimes judged that the recording head was defective to make the recording head useless.
Anyway, even by use of a multi-nozzle head, because scanning is performed for plural times by stopping of paper feeding of the recording medium, the recording speed cannot be lowered, and also, when only a recording medium excellent ink absorptivity is used, the disadvantages of the users by limitation of the recording will be brought about, such as the disadvantage of failing to use a diversity of recording media.
Further, as the problem of fixability, boundary region, the following facts have been found out, and it has been found that the difference in recording characteristics becomes marked when using different recording media.
More specifically, a great difference in ink absorption ability occurs at the boundary portion of the scanning line in overlapped recording, whereby density irregularity due to nonuniform ink absorption is liable to be generated.
Also, in the constitution which performs recording for plural times on the same picture element through the same discharge port, irregularity in density and shooting precision between the discharge ports may be sometimes intensified to lower the picture element.
Further, since the ink densities change at the same time over the whole region of the width of the discharge port (scanning region), and therefore stretching and shrinkage of the recording medium accompanied with ink absorption will occur at the same time, whereby the change amount becomes greater.
On the other hand, divided recording is a system which is to be used for a recording medium inferior in absorption characteristic, and even after the final scanning (ink emission), the recorded surface cannot be touched before elapse of a certain time. Therefore, the members such as discharge rollers, press rollers, etc. are required to be departed from the above-mentioned recording region by the head width or more, whereby enlargement of the device or lowering in paper feeding precision may be brought about.
Further, the problem of stretching and shrinkage of the recording medium as mentioned above also occurs in divided recording.